The present invention relates generally to electrodes for electrochemical cells, and particularly to electrodes comprising a transition metal and an alkali metal incorporated into a polymer wherein the transition metal is homogeneously dispersed at the nano-scale within the polymer matrix with no crystalline structure detectable and the transition metal oxide has been formed directly in the polymer.
Description of Related Art
All-solid-state thin-film lithium/polymer rechargeable batteries have been the subject of active research and development in both academia and industry. Slow transport of Li.sup.+ in the Mn oxide structure is often cited as a reason for the intrinsically poor charge/discharge performance of manganese-based positive electrodes in lithium/polymer rechargeable batteries. The main advantages of the all-solid-state polymer electrolyte cell over the competing lithium-ion battery technology, which utilizes a liquid organic electrolyte, is the overall stability of the electrolyte. The liquid electrolyte in the lithium-ion cell can evaporate on overheating due to over discharge or other mishandling. There is serious danger of ignition in the presence of high surface area metallic lithium which can form on the carbon negative electrode during the same mishandling episodes. Generally, 3V positive electrode materials, such as Li.sub.2 Mn.sub.4 O.sub.9, Li.sub.4 Mn.sub.5 O.sub.12, V.sub.2 O.sub.5, LiTiS.sub.2, and LiMoO.sub.2 are suitable for use with polymer electrolytes.
Manganese oxides are of special interest because they are environmentally benign and low cost. Oxides such as Li.sub.2 Mn.sub.4 O.sub.9 and Li.sub.4 Mn.sub.5 O.sub.12 show good reversibility with capacities of about 200 mAh/g. Various methods are currently used to create positive electrodes from manganese materials. U.S. Pat. Nos. 4,980,251 and 5,240,794 disclose heating precursor materials comprising lithium, manganese and oxygen to form the positive electrode. U.S. Pat. No. 5,340,368 discloses mixing manganese oxide with monomers, an electronic conductor such as carbon, and lithium, then polymerizing the mixture to form the positive electrode material. U.S. Pat. No. 5,514,492 discloses dissolving a polymer in a solvent such as acetonitrile, mixing in carbon, lithium and manganese dioxide to form a slurry, and wherein the manganese dioxide is maintained in suspension after the solvent is removed. U.S. Pat. No. 5,614,331 discloses forming metal oxides, then subsequently mixing said metal oxides with a polymer solution to form electrodes. In these methods, the metal oxide is formed prior to mixing with the polymer limiting the minimum grain size that can be achieved. The manganese does not become bonded to the polymer, but rather remains in a separate phase from the polymer. In addition, good cycle life is observed only when these materials are charged and discharged at fairly low current densities (e.g. less than 0.5 mA/cm.sup.2), due to lithium-ion transport limitations in the spinel structure. One solution to this problem is to decrease the particle size (thereby increasing the particle surface area) of the manganese and the polymer, shortening the solid-phase diffusion path. In addition to manganese, other transition metals have been used as electrode materials. Current methods of dispersing metal oxides (in powder form) in polymers do not achieve sufficiently small particle size of the transition metal in the polymer.
U.S. Pat. No. 5,523,179 discloses sulfur, polyethylene oxide and carbon dissolved or dispersed in acetonitrile to form a film for a positive electrode.